Carol is a clone of Mike
Some crazy mad scientists were working on a way to artifically create Half-Identical Twins
, and they paid Jason and Maggie for permission to experiment on the newborn Mike. Maggie was employed as the human incubator for said experiment, to cover up what was goin on. The experiment worked, sort of. While Carol didn't turn out quite as identical to Mike as they hoped, she was remarkably close. Afterward, the scientists moved on, and nobody in the Seaver clan spoke of the incident ever again.
- Or, being siblings they just look a lot alike.